1. Field of the Invention
Exemplary aspects of the present invention relate to a fixing device and an electrophotographic image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination thereof, and more particularly, to a fixing device for fixing a toner image on a recording medium, and an image forming apparatus including the fixing device.
2. Description of the Related Art
Related-art image forming apparatuses, such as copiers, facsimile machines, printers, or multifunction printers having at least one of copying, printing, scanning, and facsimile functions, typically form an image on a recording medium according to image data. Thus, for example, a charging device uniformly charges a surface of an image carrier; an optical writer emits a light beam onto the charged surface of the image carrier to form an electrostatic latent image on the image carrier according to the image data; a development device supplies toner to the electrostatic latent image formed on the image carrier to make the electrostatic latent image visible as a toner image; the toner image is directly transferred from the image carrier onto a recording medium or is indirectly transferred from the image carrier onto a recording medium via an intermediate transfer member; a cleaner then cleans the surface of the image carrier after the toner image is transferred from the image carrier onto the recording medium; finally, a fixing device applies heat and pressure to the recording medium bearing the toner image to fix the toner image on the recording medium, thus forming the image on the recording medium.
The fixing device used in such image forming apparatuses may include a pair of looped belts or rollers, one being heated by a heater for melting toner (hereinafter referred to as “fixing member”) and the other being pressed against the fixing member (hereinafter referred to as “pressing member”). In a fixing process, the fixing member and the pressing member meet and press against each other, forming a so-called a fixing nip through which a recording medium is passed to fix a toner image thereon under heat and pressure.
As the fixing member, a belt-type fixing member having a low heat capacity is used to reduce rise time. Although advantageous, it is difficult to adjust the temperature of such a fixing member evenly across the fixing member because, when heated, the temperature of the fixing member having a low heat capacity tends to change rapidly.
In order to obtain a desired temperature of the fixing member reliably, for example, in one related-art fixing device, a temperature detector is provided to detect the temperature of the heater and readings therefrom used to adjust the temperature of the fixing member. However, because the temperature of the fixing member is not detected directly but instead is detected indirectly via the heater, there is a delay in adjustment of the temperature of the fixing member and hence it is difficult to adjust the temperature of the fixing member evenly across the fixing member in a timely manner.
To counteract the delay, the temperature of the fixing member may be detected on the rear side of the fixing member. In this configuration, a contact-less temperature detector is used, to prevent the fixing member from getting damaged by the temperature detector. Disadvantageously, such a contact-less temperature detector is generally expensive.
In another example of detecting the temperature of the fixing member, when using a ceramic heater as a heat source, multiple temperature detectors for detecting the temperature of the ceramic heater are disposed in a recording medium passing area defined on the fixing member and through which a recording medium is conveyed. More specifically, the temperature detectors are disposed on the rear side of the ceramic heater at a center of the recording medium passing area over which any size of the recording medium is subjected to pass and at an end portion of the recording medium passing area over which a largest size of the recording medium passes.
In this configuration as well, the temperature of the fixing member is adjusted indirectly by detecting the temperature of the rear side of the ceramic heater. As a result, temperature ripple of the fixing member tends to be significant, complicating efforts to adjust the temperature of the fixing member reliably.
In order to reduce the temperature ripple of the fixing member, it is desirable to adjust the temperature of the fixing member by detecting the temperature of the fixing member directly. However, as described above, direct detection of the fixing member may cause damage to the fixing member, or the temperature detector is expensive. To address such a difficulty, detection of the temperature of the heater is proposed instead. Disadvantageously, however, if the temperature of the heater is detected at a distance while rotation of the fixing member is halted, detection of the temperature thereof is delayed, causing overheating. Furthermore, if the temperature of the fixing device is adjusted indirectly, that is, the temperature of the fixing device is adjusted by adjusting the temperature of the heater, there is a delay in transmission of heat, thereby increasing temperature ripple.
Moreover, detection of the temperature of the fixing member at a place other than a place contacting the heater also causes a delay in detection of the temperature of the fixing member when the fixing member is not rotated. That is, because the fixing member is generally made thin to have a small heat capacity to enable quick start, heat transmission in a circumferential direction thereof is small. Hence, there is a significant temperature difference in the fixing member between the contact place contacting the heater and the non-contact place.
In order to detect the temperature of the fixing member properly and reliably when the fixing member is still, it is desirable to have a temperature detector at the contact place where the fixing member comes in contact with the heater. Furthermore, when a recording medium having a narrow width is fed into the nip, the temperature of the fixing member at a non-recording medium passing area is overheated because there is no recording medium to absorb the temperature of the fixing member at that portion of the fixing member.
In view of the foregoing, it is desirable to provide a plurality of heaters to change heat generation distribution over the width of the fixing member to accommodate recording media sheets in different sizes. In this configuration, the temperature detectors need to be disposed at each heat-generating area to adjust temperature of the heat-generating areas independently.
There is a drawback to this configuration in that a plurality of temperature detectors is needed. More specifically, at least four temperature detectors are needed to detect the temperature of the center and lateral end portions of the heater in the width direction and in the circumferential direction as well at as places other than where the heater is disposed.